1. Field of the Invention
The present invention relates to methods and devices for calibrating the mobility axis of an ion mobility spectrum and for determining the mobility characteristic of ion species from an ion mobility spectrum, in particular for calibrating the drift time axis of an ion mobility spectrum acquired by a drift type ion mobility spectrometer (IMS).
2. Description of the Related Art
Ion mobility spectrometry is based on characterizing chemical substances by the gas-phase mobility of their ionic species under the influence of an electric field. It has been known as an analytical technique since the late 1960s and early 1970s. Ion mobility spectrometers (IMS) operated at ambient pressure are highly sensitive for detecting substances at low concentrations in ambient air and from vaporized samples, and have been successfully utilized for the detection of environmental pollutants, explosives and illicit drugs in the civil sector as well as for the detection of chemical warfare agents (CWAs) in the military sector.
Drift-type IMS are most commonly used in commercial instruments and are based on following principles: a gas sample or vapor from a heated sample is introduced into an ion source with an ionization region to form ions. The ions are introduced from the ion source into a drift region in a pulsed or modulated manner and migrate under the influence of a homogeneous static electric field through a drift tube, normally against a counter flow of dry carrier gas. An ion detector provided at the end of the drift tube is used to measure the drift time it takes for the ionic species to pass through the drift tube. There are other types of IMS operated at ambient pressure, for example Differential Mobility Spectrometry (DMS, also known as Field Asymmetric Ion Mobility Spectrometry, FAIMS) and the aspiration-type IMS.
The gas-phase substances from a sample source (e.g., a desorber or direct sampling of ambient air) are commonly introduced by a carrier gas into the ion source of the IMS and are most commonly ionized by chemical ionization (CI). The carrier gas of an IMS is typically purified air with only some parts per million (ppm) of water vapor. Electrons emitted for example by a radioactive beta emitter, such as Ni63, generate positive nitrogen ions by electron impact ionization and negative oxygen ions by electron attachment of thermalized electrons. The nitrogen and oxygen ions further react with water molecules present as vapor in the carrier gas to generate positive (H+(H2O)n) or negative water cluster ions (O2−(H2O)n), respectively. These secondary reactant ions react with gas-phase substances by protonation forming positive product ions, or by adduct formation forming negative product ions. The primary oxygen ions may also react with gas-phase substances by de-protonation, electron transfer or adduct formation forming negative product ions. The electrons generating the primary reactant ions can be provided by radioactive as well as by non-radioactive electron sources, such as electrical discharges, electron beam generators and/or UV/X-ray lamps. However, direct ionization of the gas-phase substances by UV and X-ray is also possible.
It is well known that, for drift-type IMS, the measured drift time td of an ion species can be calculated in a good approximation from following equation:
      t    d    =                    L        /        E            ·                        T          0                /        T            ·              P        /                  P          0                            K      0      wherein L is the length of the drift tube, E is the homogeneous static electric field strength in the drift tube, T0 is the standard temperature, T is the actual temperature in the drift tube during the measurement, P is the actual pressure in the drift tube during the measurement, Po is the standard pressure and K0 is the reduced mobility of the ion species. The physical characteristic of an ion species together with the properties of the drift gas are combined into the reduced mobility K0 which corresponds to the mobility of the ion species in the drift gas at standard conditions of temperature and pressure. A large value of reduced mobility equates to rapid motion of the ion species in the electric field and thus to a small cross section for collisions with the drift gas. The reduced mobility is the characteristic mobility measure of the ion species to be determined from the measured drift time and experimental parameters. If the aforementioned equation is rearranged to K0 as function of the drift time and experimental parameters, it can serve as a calibration function between the drift time axis of a measured ion mobility spectrum and the mobility axis of a calibrated ion mobility spectrum.
The length of the drift tube and the electric field strength of a drift type IMS are fixed or controlled, respectively. The actual pressure P and temperature T during the measurement are further experimental parameters which need to be determined in order to calculate the reduced mobility K0 from the measured drift time. The temperature can be measured with a simple and inexpensive sensor with reasonable accuracy, whereas measuring the pressure inside the drift tube with a sufficient accuracy needs quite elaborate pressure gauges. The determined experimental parameters can be used to establish a calibration function.
Besides calculating the reduced mobility from the measured drift time and the actual experimental parameters during the acquisition of the ion mobility spectrum, it is well known to utilize a calibrant that is introduced into the ionization region of the IMS ion source together with gas-phase substances from the sample source. The reduced ion mobility of the calibrant ions is known and the drift time of calibrant ions is measured under the actual conditions of the drift type IMS. Under the reasonable assumption that the experimental conditions seen by the calibrant ion and the sample ions are similar through the ion source and drift region, the reduced ion mobility of sample ions can be determined from the measured drift time of the sample ions using the measured drift time of the calibrant ions and their known reduced mobility.
FIG. 1 shows a schematic drawing of an ion mobility spectrometer 1 known from the prior art. The ion mobility spectrometer 1 comprises an ion source 10, an ion mobility analyzer 20 (e.g., a drift type analyzer) fluidly coupled to the ion source 10 and a calibrant reservoir 30. An air sample (or more generally a gas sample to be analyzed) is drawn by gas pump 40 into the ion source 10 at a first inlet 51a from the surroundings. The ion mobility spectrometer 1 comprises an additional inlet 51b and a gas outlet 52. If valve 31 is opened, air from outside is also drawn into calibrant reservoir 30. The gas-phase sample and the calibrant are ionized in the ionization region 11 by a radioactive beta emitter 12 and transferred to the mobility analyzer 20, e.g., by the gas flow generated by gas pump 40. Alternatively, the gas-phase sample and calibrant can be mixed prior to the ion source and introduced into the ion source 10 at a single inlet.
In general, a calibration function provides a relation between the abscissa of a measured ion mobility spectrum (e.g., the drift time in a drift type IMS or the compensation voltage in FAIMS) and a mobility characteristic (e.g., the reduced mobility). The ion mobility generally depends on the actual temperature and pressure in the mobility analyzer. Therefore, the calibration function commonly depends on the actual temperature and pressure.
The international patent application WO 2009/088461 A2 provides an explosive and narcotics detection system using an ion mobility spectrometer. The calibration of the spectrometer depends on the stability of the used calibrant 5-nitrovanilin that is periodically injected together with sample gas into the ionization region of the spectrometer. The calibrant produces a signal with a known drift time and may be used to calibrate the drift times of the sample ions.
The international patent application WO 2010/051241 A1 teaches that calibrants and reactants used in conventional ion mobility spectrometry systems can impair the ability to detect species of interest by suppressing the signal amplitude of the species of interest or having coincident drift times, for example. In order for the IMS to have good sensitivity for the weakly ionized substances, the reactant/calibrant must not interfere with the ionization process or mask the ion's unique drift-time peaks.
The international patent application WO 2008/053181 A1 provides a FAIMS being arranged so that the analyte is subject to different ion chemistries at different locations along the spectrometer.
In view of the foregoing, there is still a need to provide devices for fast and reliable delivery of calibrant ions and methods for accurately calibrating the mobility axis of an ion mobility spectrum and determining the mobility characteristic of ion species from an ion mobility spectrum.